Optical microcavities are known to confine light to a small volume. Devices using optical microcavities are today essential in many fields, ranging from optoelectronics to quantum information. That is, they are key components for lasers, optical filters, optical sensors and devices for optical quantum computing and simulations. Typical applications are long-distance data transmission over optical fibers and read/write laser beams in DVD/CD players. A variety of confining semiconductor microstructures has been developed and studied, involving various geometrical and resonant properties. A microcavity has smaller dimensions than a conventional optical cavity; it is often only a few micrometers thick and the spacer layer that it comprises can even reach the nanometer range. Such dimensions notably allow for studying quantum effects of electromagnetic fields.
In more detail, a cavity or a microcavity forms an optical cavity or resonator, which allows for a standing wave to form inside the spacer layer. The light emission is perpendicular to the substrate plane. The thickness of the spacer layer determines the cavity-mode, which corresponds to the wavelength that can be transmitted and forms a standing wave inside the resonator. An ideal cavity would confine light indefinitely (that is, without loss) and would have resonant frequencies at defined values. The deviations from this ideal paradigm are captured by the cavity Q factor, which is proportional to the confinement time in units of the optical period. However, some losses are actually required, because otherwise, e.g., no laser light could be outcoupled or no light could be filtered. Now, other parasitic loss channels than the intended one should be limited as much as possible. For instance, one may deliberately choose a mirror reflectivity below 100%, e.g., only 99%, to make sure that light leaves the cavity only in a certain direction, through this mirror and not through scattering losses (in all directions).
Another important descriptive parameter is the effective mode volume (V), which relates to the optical modes present in the cavity. Every mode has a certain mode volume, i.e., the spatial volume which will be filled with photons when a mode is excited. So the mode volume is a property of each and every mode (and will differ between the modes). Vertical cavities (including cavities as contemplated in the detailed description below) preferably support only a single mode or a few modes that would all have a small effective mode V. Such a configuration is indeed desirable for most applications.
Accordingly, the realization of practical devices requires maximizing the ratio Q/V, i.e., high values for Q and low values for V are important to increase light-matter interactions in processes such as spontaneous emission, lasing, nonlinear optical processes and strong coupling.
More in details, the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is. The value of Q is usually defined as 2πX the total energy stored in the structure, divided by the energy lost in a single oscillation cycle. A high quality factor Q and a small mode volume V are desirable for many applications but are hard to reach simultaneously.
Vertical microcavity designs have been proposed, wherein a light confinement region is defined between reflectors, where the confinement region comprises a “defect”, e.g., (i) a disk-shaped structure (e.g., forming an aperture), formed from an absorbing material or a metal; (ii) a mesa (e.g., of a dielectric/semiconductor material); or (iii) a 3D-shaped defect (e.g., formed from a dielectric material). The first two designs, however, provide low Q factors, typically less than 104, whereas 3D-shaped defects requires 3D lithography.